One of construction methods for producing a shaped raw material is a method referred to as powder metallurgy. In the powder metallurgy, metal powders as starting materials are mainly bonded to each other according to a sintering phenomenon, to produce a material. The powder metallurgy is used for shaped raw material production in a wide range of areas such as a magnetic material, an ultrahard alloy, a mechanical structure application part, atomic fuel, and ceramics. Examples of the feature of the powder metallurgy include the following items (1) and (2): (1) an alloy composition can be freely controlled by merely changing the combination or the blending ratio of starting material powders; and (2) the powder metallurgy has a simple step of heating a molded body having a shape almost close to an end product shape to provide an excellent material yield ratio and less consumption energy.
The physical properties of a material obtained by powder metallurgy are largely influenced by the fine structure formation or the composition of the material. In recent years, a small amount (1% or less) of an addition element, a micro structure of 0.1 μm or less, or arrangement of an atomic level, or the like frequently influences the physical properties of the material largely. The production method of the target material or the control method of the material physical properties becomes important. To that end, it is important to understand a change in a structure occurring in the material at any stage during sintering and a heat treatment. Particularly, from the understanding of the movement of an atom through a liquid phase in liquid phase sintering or the like, the optimal process can be considered in order to obtain the target material physical properties or achieve the target fine structure.
One of methods for observing the fine structure is an atom probe. The atom probe can directly observe atomic arrangement or composition distribution of a leading end of a material which has been processed so as to be needle-shaped, at an atomic scale. In the atom probe, a high direct current voltage is applied so as to cause the leading end of the needle-shaped material to generate a high electric field. A pulse voltage is applied or a pulse laser is irradiated to the leading end so that field evaporation of an atom belonging to a first layer of a surface is induced. Then, the mass of an ion which has field-evaporated is time-of-flight-measured so that a type of an element can be determined. The field evaporation progresses every atomic layer. For this reason, atom probe analysis has resolution in a depth direction at an atomic level.
Since the atom probe is time-of-flight type analysis, the kind of the element is recognized by a ratio of mass to valence (=mass charge ratio). For this reason, a system in which an element A and an element B having a mass number of constant multiple (about 2 or 3 times) thereof are mixed makes it difficult to perform quantitive identification of both the elements. For example, the analysis of silicon (Si) having a mass number of 28 contained in iron (Fe) having a mass number of 56, or the analysis of carbon (C) having a mass number of 12 contained in magnesium (Mg) having a mass number of 24, or the like is difficult.
On the other hand, the atom probe analysis can distinguish the same elements (isotope) having different mass numbers. Therefore, if an isotope of nitrogen having a mass number of 15 is introduced into a material in order to analyze nitrogen (N) having a mass number 14 contained in silicon (Si) having a mass number of 28, the weak point of the atom probe analysis can be overcome (NPL 1). Incidentally, in elemental analysis according to fluorescence or Auger electrons such as EDX (energy dispersion type X-ray analysis), the same elements having different mass numbers cannot be distinguished.